1. Field of the Invention
The present invention relates generally to fluid infusion systems and more particularly to an improved flow-metering apparatus for forcibly introducing a fluid into a positively pressured system such as the human body.
2. Description of the Prior Art
There are numerous types of pumps used to forcibly infuse liquids into the human body. Most of these pumps are of the peristaltic type wherein a round flexible tube (lumen-defining member) filled with the fluid is milked by continuous compression of the tube walls toward each other in a wavelike motion along a length of the tube to produce the pumping action. These peristaltic pumps rely on the memory of the elastomeric materials making up the tube walls to reset the internal volume, to determine pumping accuracies, and to provide reinflation power to the pump to draw more fluid from the reservoir to be pumped into the patient.
A number of problems have been created by the peristaltic nature of these pumps and further from the continuous compression-release cycle applied to the flexible tubing. The lumen-defining member is typically a cylindrical tube made of silicone rubber that demonstrates variable rebound characteristics with temperature and age. Peristaltic pumping causes significant stress to be placed on the elastomeric tube wall. Repeated compression-release cycling raises the stress at the area of the most abrupt change in curvature, namely the areas along the sides of the tube transiting the upper and lower wall segments. This repeated stress of pumping causes small bits of wall material to flake off and enter the stream of flowing fluid to either pass into the patient's body as a foreign substance or create an occlusion down stream to interrupt or diminish the flow of fluid thereby ruining the accuracy of the peristaltic pump.
Still further, the total collapse or pressing together of the lumen-defining member during the peristaltic pump action causes some compression of stream material between those wall sections. While homogeneous fluids are not greatly affected by this action, other types of fluids that include minute entrainments such as blood cells in a blood transfusion will cause cell damage and thereby deteriorate the quality of the infused material. Finally, these peristaltic pumps are very complex and include numerous parts that must be maintained freely mobile in the pump assembly to bring about the peristaltic action. Any leakage of fluid into these mechanisms thereby poses substantial sticking and malfunction of the pump components. The peristaltic pump components are susceptible to damage from spills of medicinal liquids either through cracking of the lumen-defining member or whenever the bag from which the contents are drawn breaks or cracks.